The one handed sharps are the weapons in the top row of the primary weapons in the Man-At-Arms class and the second row of the secondary weapons in the knight class. To get this achievement you will need to get 25 kills with the first one handed sharp weapon that is unlocked.
I personally haven't noticed any preference in game mode for these weapons as they are a good all rounder. To make this a bit easier you should accompany your weapon with a shield as it makes blocking easier.
Any suggestions for this solution just comment. :D